0:00:08 > 0:00:12It insulates our planet from the cold hostility of space...
0:00:18 > 0:00:21..and shields us from the sun's deadly rays.
0:00:25 > 0:00:28It brings live-giving water...
0:00:32 > 0:00:34..and it's in every breath you take.
0:00:41 > 0:00:43It is our atmosphere.
0:00:48 > 0:00:51Now, a team of scientists are going on an expedition...
0:00:53 > 0:00:58..to explore this elusive and precious realm.
0:00:58 > 0:01:01We have this dynamic bubble of air
0:01:01 > 0:01:04constantly moving, constantly changing
0:01:04 > 0:01:07and that's what we are here with Cloud Lab to explore.
0:01:08 > 0:01:12This unique laboratory, an airship 200 feet long,
0:01:12 > 0:01:16is packed with the latest scientific instruments.
0:01:16 > 0:01:18Scan it up and down vertically and see if we can hit it.
0:01:18 > 0:01:21It actually goes right up to the sun level.
0:01:21 > 0:01:25It will enable the team to carry out ground-breaking experiments...
0:01:30 > 0:01:33This is really good - now we are sucking in the clouds.
0:01:33 > 0:01:37..to discover the many surprising ways in which the atmosphere
0:01:37 > 0:01:39shapes our world.
0:01:41 > 0:01:42From the edge of the jet stream...
0:01:48 > 0:01:50..to the bottom of the ocean.
0:01:54 > 0:01:59Another giant-sized animal. This whole place is, like, super-sized.
0:02:02 > 0:02:07And the ways in which we ourselves are changing the atmosphere.
0:02:07 > 0:02:12We've hard evidence that human beings are creating their own weather.
0:02:28 > 0:02:31An airstrip in south-east Florida,
0:02:31 > 0:02:34and the team get their first sight of the airship.
0:02:36 > 0:02:39It's a lot bigger than I thought it would be. I genuinely thought...
0:02:39 > 0:02:43I've been on expeditions in some pretty extraordinary vehicles
0:02:43 > 0:02:46but this has got to beat the lot, surely?
0:02:48 > 0:02:51This is better than my normal lab, by a long way.
0:03:05 > 0:03:08With an expertise in meteorology,
0:03:08 > 0:03:12the expedition is being led by explorer Felicity Aston.
0:03:14 > 0:03:18I've spent lots of time looking at the weather from the ground
0:03:18 > 0:03:21and seeing satellite pictures taken from above the atmosphere,
0:03:21 > 0:03:24but with this we're going to be able to actually go into the clouds
0:03:24 > 0:03:27and see the weather from the inside.
0:03:27 > 0:03:32So you can't help but be excited about something like this.
0:03:33 > 0:03:37The team will fly the airship coast to coast across America,
0:03:37 > 0:03:40from the Atlantic all the way to the Pacific.
0:03:42 > 0:03:44The extreme range of atmospheric conditions
0:03:44 > 0:03:47this continent encompasses
0:03:47 > 0:03:50will enable them to investigate three distinct themes...
0:03:52 > 0:03:57Life. They want to discover the many complex ways in which wildlife
0:03:57 > 0:04:00exploits every level of the atmosphere,
0:04:00 > 0:04:04from close to the earth's surface to the death-zone of high altitude.
0:04:08 > 0:04:13Human impact. They'll explore the subtle and surprising ways
0:04:13 > 0:04:15in which we change the atmosphere.
0:04:23 > 0:04:26And weather. The many extraordinary processes
0:04:26 > 0:04:30that generate weather in the atmosphere.
0:04:30 > 0:04:33MAN ON RADIO: Clear for take-off, remain south, runway 9.
0:04:33 > 0:04:37And this is where their journey begins -
0:04:37 > 0:04:40with one of the most beautiful, transitory and mysterious
0:04:40 > 0:04:43of all weather phenomena - clouds.
0:04:46 > 0:04:49- Fans?- Fans are on.
0:04:54 > 0:04:58Felicity will examine how clouds capture and store liquid water
0:04:58 > 0:05:01in the skies to form an ocean of water above our heads.
0:05:01 > 0:05:04That's the one I want! That one.
0:05:06 > 0:05:09- Good for departure.- Temperature pressures.- Green for departure.
0:05:09 > 0:05:13Andy Torbet will measure the forces within clouds
0:05:13 > 0:05:16that keep this water floating in the sky.
0:05:25 > 0:05:29Dr Chris van Tulleken will see exactly how clouds
0:05:29 > 0:05:31return water to earth
0:05:31 > 0:05:35by unravelling one of the remaining mysteries of meteorology -
0:05:35 > 0:05:36what makes raindrops form?
0:05:38 > 0:05:42We know about soot, sand and dirt
0:05:42 > 0:05:47but I'm looking for something a bit different.
0:05:47 > 0:05:51Which is why, as an infectious diseases doctor, I'm up in a cloud.
0:05:57 > 0:06:02Backing up the team is atmospheric chemist Jim McQuaid,
0:06:02 > 0:06:04who's custom-built the lab.
0:06:04 > 0:06:08The instrumentation we've got here will measure gases in the atmosphere,
0:06:08 > 0:06:09pollution, particle measurement.
0:06:11 > 0:06:14This is a laser system that will measure clouds off in the distance.
0:06:14 > 0:06:18And then we can measure sunlight on the top and also on the bottom
0:06:18 > 0:06:21of the airship. So, we've got a really nice little set
0:06:21 > 0:06:25of measurements that will allow us to explore the atmosphere.
0:06:31 > 0:06:35Sharing the journey will be a 15-strong support team,
0:06:35 > 0:06:38needed to launch one of the biggest airships in the world.
0:06:57 > 0:07:00The flying capabilities of the airship
0:07:00 > 0:07:03offer the team a unique research platform,
0:07:03 > 0:07:04able to conduct experiments
0:07:04 > 0:07:07that would be impossible in any other aircraft.
0:07:09 > 0:07:12They're beginning their expedition with clouds,
0:07:12 > 0:07:15because without them we simply wouldn't be here.
0:07:21 > 0:07:23It's difficult to imagine,
0:07:23 > 0:07:27but the skies are home to a vast ocean of water.
0:07:27 > 0:07:30Yet it is beyond our reach,
0:07:30 > 0:07:35suspended all around us as an invisible vaporous gas.
0:07:37 > 0:07:42Only once it is transformed into clouds does it become liquid water.
0:07:43 > 0:07:46It's this deceptively simple transformation
0:07:46 > 0:07:48of water from gas to liquid
0:07:48 > 0:07:50that ultimately brings water
0:07:50 > 0:07:52from the sea to the earth's land surfaces,
0:07:52 > 0:07:58by generating 1.4 trillion tonnes of rainfall every day.
0:08:02 > 0:08:06Yet clouds are as mysterious as they are beautiful.
0:08:08 > 0:08:13How can such delicate ephemeral structures carry so much water?
0:08:19 > 0:08:23To begin to understand exactly how much water they carry,
0:08:23 > 0:08:27Felicity wants to try something that's never been attempted before.
0:08:30 > 0:08:32So, what would be really great,
0:08:32 > 0:08:34I don't know if it's going to be possible or not,
0:08:34 > 0:08:37but what would be really great is if we could weigh a cloud,
0:08:37 > 0:08:41see how heavy it is and work out how much water is in one of those clouds.
0:08:42 > 0:08:45But to do that, we've got to get up there.
0:08:45 > 0:08:47So we've got to do a bit of cloud hunting.
0:08:58 > 0:09:02The Florida coastline is the perfect place to hunt for clouds,
0:09:02 > 0:09:04because it's in the ocean
0:09:04 > 0:09:07where water's journey into the atmosphere begins.
0:09:09 > 0:09:14Energy from the sun evaporates water from the sea into the air above,
0:09:14 > 0:09:16and when this moist air is warm enough,
0:09:16 > 0:09:20it starts to rise in a column of air known as a thermal.
0:09:22 > 0:09:25As it rises, it gets colder.
0:09:25 > 0:09:29And cold air can't hold as much water as warm air.
0:09:29 > 0:09:33So you get to a certain level when it's cold enough,
0:09:33 > 0:09:36that all that water from the sea starts to rematerialize
0:09:36 > 0:09:41as tiny little droplets of water. That is the birth of a cloud.
0:09:51 > 0:09:55OK, we're going to go for a cloud...
0:09:55 > 0:09:57Unlike other aircraft,
0:09:57 > 0:10:00the airship can travel slowly enough inside the cloud
0:10:00 > 0:10:03to take the crucial measurements Felicity will need.
0:10:03 > 0:10:07OK, these clouds here are a little bit wispy and broken.
0:10:10 > 0:10:13These ones look as if they are towering a bit too much.
0:10:13 > 0:10:17- I think that one's lower, over there, you know.- This one here?
0:10:17 > 0:10:19OK, that's the one I want. That one.
0:10:19 > 0:10:23It'll be really great to go right through the middle
0:10:23 > 0:10:24and right into the heart of it.
0:10:26 > 0:10:30This is the airship November 6-1-0 Sierra Kilo...
0:10:30 > 0:10:34It will take all of chief pilot David Byrne's 30 years of experience
0:10:34 > 0:10:37to reach the target cloud in time.
0:10:37 > 0:10:416-1-0 is an airship. We'd like to operate in this area
0:10:41 > 0:10:43between 2,500 and 3,000 feet.
0:10:43 > 0:10:46MAN ON RADIO: Sierra Kilo, roger. Proceed as requested.
0:10:49 > 0:10:54Small cumulus clouds like this last on average just ten minutes,
0:10:54 > 0:10:56so they'll need to move fast.
0:11:03 > 0:11:06Meanwhile, former paratrooper Andy Torbet
0:11:06 > 0:11:09is preparing for the team's second mission,
0:11:09 > 0:11:12researching another aspect of clouds -
0:11:12 > 0:11:14what keeps them in the air?
0:11:22 > 0:11:24To do so, he'll be travelling through a cloud
0:11:24 > 0:11:28in a way that the airship can't - vertically.
0:11:30 > 0:11:32The plan is to find a nice cloud, one that's growing,
0:11:32 > 0:11:35one that's sucking moisture up from the surface of the earth,
0:11:35 > 0:11:38and to get out the aircraft 1,000 feet above the top of that,
0:11:38 > 0:11:40and then drop just beneath it
0:11:40 > 0:11:43and then fly my parachute just under the cloud.
0:11:48 > 0:11:51Whilst a thermal is enough to give birth to a cloud,
0:11:51 > 0:11:55for it to remain in the air it needs another source of energy.
0:11:57 > 0:12:00That energy comes from within the cloud itself.
0:12:00 > 0:12:03As molecules of water vapour come together in a cloud,
0:12:03 > 0:12:07they release the heat absorbed during evaporation.
0:12:10 > 0:12:13And it's this heat energy that Andy is hoping to detect.
0:12:17 > 0:12:20So as he descends through the cloud
0:12:20 > 0:12:23he'll record a continuous stream of temperature readings.
0:12:32 > 0:12:34It's an experiment fraught with hazard.
0:12:37 > 0:12:40The powerful air currents that thrust the cloud upward
0:12:40 > 0:12:41also generate turbulence.
0:12:43 > 0:12:46Normally with skydiving you look to avoid clouds.
0:12:46 > 0:12:50This is the first time I'll be going to aim and hit a cloud.
0:12:50 > 0:12:53To mitigate the risks, the entire experiment is being supervised
0:12:53 > 0:12:57by a skydive master, Dane Kenny.
0:12:57 > 0:13:00OK, Andy, 1,500 feet, throw the scientific stuff,
0:13:00 > 0:13:04we start thinking about landing, downwind to make the final leg.
0:13:04 > 0:13:07- Roger.- Happy?- Happy.
0:13:07 > 0:13:11Dane will try to find a route for Andy through the edges of the cloud,
0:13:11 > 0:13:15where he can detect the release of heat whilst still remaining safe.
0:13:26 > 0:13:27Aboard the airship,
0:13:27 > 0:13:30they're closing in on the cloud they've targeted for weighing.
0:13:34 > 0:13:37Dr Jim McQuaid primes the instrumentation.
0:13:39 > 0:13:42So we have a... There's a laser beam here.
0:13:42 > 0:13:44So this is one instrument we've got, it's called a LIDAR.
0:13:47 > 0:13:51The LIDAR, a kind of light radar, will measure the cloud's dimensions
0:13:51 > 0:13:56by emitting a laser and analysing the light reflected back.
0:13:56 > 0:13:59So, the time it takes for the light to go from here
0:13:59 > 0:14:02to the cloud and back will tell us the distance.
0:14:04 > 0:14:08A second probe will measure the exact size and density
0:14:08 > 0:14:10of the individual droplets of liquid
0:14:10 > 0:14:13as the airship passes through the cloud.
0:14:14 > 0:14:17- OK, Jim, are you ready?- OK!
0:14:31 > 0:14:32So, I'm picking up cloud droplets now.
0:14:32 > 0:14:34The humidity's gone up to 100%.
0:15:11 > 0:15:13I don't know how many times, as a kid,
0:15:13 > 0:15:17I wondered what it would feel like to be up in one of these clouds.
0:15:17 > 0:15:20And now I've just gone through one, so now I know.
0:15:20 > 0:15:23It doesn't feel like cotton wool sadly, but...
0:15:25 > 0:15:28It feels really wet and surprisingly dark in there.
0:15:38 > 0:15:40That was great, that was really perfect.
0:15:40 > 0:15:43With the moist, cool air of the cloud behind them,
0:15:43 > 0:15:45they can begin to figure out the result.
0:15:45 > 0:15:49Wow, so that cloud was nearly a kilometre long.
0:15:49 > 0:15:52So, Jim, have you got an idea of how wide the cloud was?
0:15:54 > 0:15:55200 metres across.
0:15:55 > 0:15:59So we're going to assume that it's as tall as it is wide,
0:15:59 > 0:16:02because it looked like a fairly solid elliptical shape,
0:16:02 > 0:16:06so we just use a simple formula to work out the volume of the cloud.
0:16:06 > 0:16:08How wide was it, 200 metres?
0:16:08 > 0:16:12- 20 million.- 20 million... - ..cubic metres.
0:16:12 > 0:16:17That's a small, compact cloud, 20 million cubic metres.
0:16:20 > 0:16:23To calculate the cloud's weight,
0:16:23 > 0:16:26they factor in the size and density of the water droplets within it.
0:16:32 > 0:16:36The weight per cubic metre is about...
0:16:36 > 0:16:38Say the average is 0.2.
0:16:38 > 0:16:42- 0.2g per cubic metre. - 0.2g per cubic metre.
0:16:42 > 0:16:46- OK, so we times 0.2 by 20 million.- Yes.
0:16:48 > 0:16:504,000kg.
0:16:50 > 0:16:55- Yeah.- So that small cloud weighs four tonnes.- Yeah.
0:16:55 > 0:16:57- That's incredible.- It is.
0:16:57 > 0:17:00And that was a small one.
0:17:00 > 0:17:03Well, I think we can congratulate ourselves. We've weighed a cloud.
0:17:03 > 0:17:05We know it weighs four tonnes.
0:17:05 > 0:17:08I don't know if anybody has ever done it before.
0:17:08 > 0:17:10I'm not sure anyone's going to believe us,
0:17:10 > 0:17:15that cloud weighs four tonnes, but it does. All the figures are there.
0:17:15 > 0:17:17Your machine did good.
0:17:22 > 0:17:25Felicity's experiment has revealed that even a small
0:17:25 > 0:17:31cumulus cloud converts large amounts of vapour to liquid water.
0:17:41 > 0:17:45It also begins to explain how, despite being fleeting,
0:17:45 > 0:17:49delicate structures, clouds can deliver all the earth's water needs.
0:17:56 > 0:18:00The average cumulus is 50 times larger than the one the team
0:18:00 > 0:18:04have measured, so it carries around 200 tonnes of water.
0:18:09 > 0:18:13Even the most diffuse cloud, a wispy, high altitude cirrus
0:18:13 > 0:18:16of the same volume, would weigh two tonnes.
0:18:22 > 0:18:27But the greatest water bearers are cumulonimbus clouds.
0:18:27 > 0:18:31Up to ten times more dense than a cumulus cloud, and measuring
0:18:31 > 0:18:36on average 1,000 times larger, these can weigh one million tonnes.
0:18:38 > 0:18:41At any one point in time, the world's clouds
0:18:41 > 0:18:47hold an astonishing 129 billion tonnes of water in the sky.
0:18:55 > 0:18:59So, given clouds carry vast amounts of water,
0:18:59 > 0:19:02they must also generate vast amounts of energy
0:19:02 > 0:19:06in order to defy gravity and remain aloft.
0:19:06 > 0:19:10Dane and Andy are seeking to measure this process, as it occurs,
0:19:10 > 0:19:14by detecting the heat energy generated by a cloud.
0:19:14 > 0:19:18They've climbed to 10,000 feet amongst a cluster of cumulus clouds.
0:19:18 > 0:19:21- Happy?- Happy.- Your handles?
0:19:21 > 0:19:22- Happy?- Happy.
0:19:28 > 0:19:30But the clouds are building fast.
0:19:30 > 0:19:33Too fast for Dane's liking.
0:19:33 > 0:19:36We're going to descend to 8,000 feet
0:19:36 > 0:19:39because there's a lot of turbulence up here, and I want to make sure
0:19:39 > 0:19:43I put Andy out in the right place at the right time.
0:19:43 > 0:19:47Dane needs to position the aircraft above a cloud, so that they can
0:19:47 > 0:19:50descend through its fringes and then beneath it.
0:19:50 > 0:19:52That will enable Andy
0:19:52 > 0:19:55to take the stream of temperature readings he needs.
0:19:57 > 0:20:00With a suitably isolated cloud in sight,
0:20:00 > 0:20:01Dane times the run-in.
0:20:11 > 0:20:17Five, four, three, two, one, go!
0:20:23 > 0:20:27They free-fall to 7,000 feet to reach the cloud tops.
0:20:31 > 0:20:34Now Andy's instruments can set to work.
0:20:42 > 0:20:44They've managed to fly into the edge of the cloud,
0:20:44 > 0:20:47and are soon met by its powerful updraughts.
0:20:49 > 0:20:52I can feel the turbulence. There's a lot of activity here
0:20:52 > 0:20:55and it's throwing my canopy about.
0:21:03 > 0:21:09I'm now at 3,800 feet and getting good readings on the Flytec.
0:21:09 > 0:21:11Excellent, mate. Good job.
0:21:15 > 0:21:18They've reached 3,000 feet and the cloud base.
0:21:18 > 0:21:21They navigate beneath it to record the way the temperature changes
0:21:21 > 0:21:23now they're out of the cloud.
0:21:30 > 0:21:33Andy, I want you to head towards the drop zone.
0:21:33 > 0:21:35You should be able to see the drop zone.
0:21:35 > 0:21:40- Head towards the sun.- Roger.
0:21:40 > 0:21:43Yeah, I can see the drop zone, so that's good.
0:21:43 > 0:21:48All Andy needs to do now to complete his data set is reach the ground.
0:21:49 > 0:21:51- Yee-ha!- Dane to Andy - down safe, mate.
0:21:57 > 0:21:59Andy has measured the air temperature
0:21:59 > 0:22:03from the top of the cloud all the way to ground level.
0:22:03 > 0:22:07It's now up to Felicity to see if they've managed to detect
0:22:07 > 0:22:10the generation of heat energy within the cloud.
0:22:12 > 0:22:16I've just been having a look at the data that came back from Andy's jump
0:22:16 > 0:22:19and they're perfect. They're exactly what we wanted.
0:22:24 > 0:22:28The data reveals that the atmosphere cools at a predictable rate,
0:22:28 > 0:22:32called the lapse rate, from ground level to the cloud base.
0:22:34 > 0:22:36But then the rate of cooling slows.
0:22:40 > 0:22:43So the cloud is clearly generating heat.
0:22:45 > 0:22:49And why that's really lovely to see that is because what we know happens
0:22:49 > 0:22:51is that when water condenses out of air
0:22:51 > 0:22:54it releases a huge amount of energy,
0:22:54 > 0:22:57and that energy warms the air around it
0:22:57 > 0:23:00and that creates big bursts of energy inside the cloud.
0:23:02 > 0:23:06And that's why clouds have big, uneven fluffy tops.
0:23:06 > 0:23:09So this is exactly what helps to keep the cloud afloat.
0:23:13 > 0:23:16This energy, released by water vapour as it condenses,
0:23:16 > 0:23:19is called latent heat,
0:23:19 > 0:23:20and it is possible to work out
0:23:20 > 0:23:25how much energy is delivered to a cloud by this process.
0:23:25 > 0:23:29A typical cumulus cloud, similar to the one Dane and Andy measured,
0:23:29 > 0:23:34generates enough heat energy to power the average home for 17 years,
0:23:34 > 0:23:37or about 300 tonnes of TNT.
0:23:45 > 0:23:49Scale that up to a million tonne cumulonimbus, and you're looking at
0:23:49 > 0:23:52the heat energy equivalent to a nuclear warhead.
0:23:56 > 0:23:59It's really great that we've managed to detect
0:23:59 > 0:24:02that release of latent heat,
0:24:02 > 0:24:03because it is so important
0:24:03 > 0:24:07to all the different weather systems that we see.
0:24:07 > 0:24:10It's the fundamental driving force, it's the energy source
0:24:10 > 0:24:13of every single weather system.
0:24:17 > 0:24:20So the formation of a cloud is not just
0:24:20 > 0:24:24the transfer of massive amounts of water to the skies,
0:24:24 > 0:24:26but massive amounts of energy, too.
0:24:28 > 0:24:31And that means clouds not only have the power to nourish our planet,
0:24:31 > 0:24:34delivering rain to the earth,
0:24:34 > 0:24:38but if that energy is released quickly, wreak destruction.
0:24:39 > 0:24:42It's a theme the team will examine further into their mission,
0:24:42 > 0:24:46but first they want to complete their study of clouds.
0:24:56 > 0:24:59The team is heading west across the Florida peninsula,
0:24:59 > 0:25:02towards the area known as the Panhandle.
0:25:22 > 0:25:25Having investigated how water arrives in our skies
0:25:25 > 0:25:27and is held aloft,
0:25:27 > 0:25:30the team have come here to examine how water
0:25:30 > 0:25:32is returned to earth in the form of rain.
0:25:35 > 0:25:37At the heart of this question
0:25:37 > 0:25:41is one of the most radical ideas in meteorology today -
0:25:41 > 0:25:44that some clouds are alive,
0:25:44 > 0:25:49and as a consequence, behave differently to others.
0:25:49 > 0:25:53- There's a hint of a wee bit of rain there.- To the north-west...
0:25:53 > 0:25:56INDISTINCT RADIO CHATTER
0:25:56 > 0:25:59It all comes down to the little-understood process
0:25:59 > 0:26:01that causes raindrops to form.
0:26:05 > 0:26:08What we are looking for is the stuff that makes rain.
0:26:08 > 0:26:11Rain doesn't form easily,
0:26:11 > 0:26:14which people in the UK and frankly, people in Florida,
0:26:14 > 0:26:17are going to think is a bit odd because it rains a lot.
0:26:17 > 0:26:22But you need a little catalyst, a nucleus, to help raindrops form.
0:26:22 > 0:26:26It's a bit like a grain of sand at the heart of a pearl.
0:26:30 > 0:26:34Molecules of water vapour need a surface to collide with,
0:26:34 > 0:26:36and condense onto, to form a liquid.
0:26:40 > 0:26:44Normally, tiny particles like dust or sea salt
0:26:44 > 0:26:46suspended in clouds do the job.
0:26:46 > 0:26:49But a new idea has emerged
0:26:49 > 0:26:52suggesting the presence of something quite different,
0:26:52 > 0:26:55which could be causing some clouds to produce rain
0:26:55 > 0:26:56while others don't...
0:26:58 > 0:27:02Life. In the form of bacteria.
0:27:05 > 0:27:08It's a theory Chris and Jim are seeking to find evidence for.
0:27:11 > 0:27:13What we are trying to find out is,
0:27:13 > 0:27:18is there bacteria in the droplets of water?
0:27:18 > 0:27:23Do they have stuff in them that could act as a nucleus to help form rain?
0:27:26 > 0:27:30As a microbiologist, Chris is used to examining bacteria
0:27:30 > 0:27:32that live within the human body.
0:27:33 > 0:27:36He's now hoping the airship's ability to enter clouds
0:27:36 > 0:27:40with minimum disturbance will enable him to see if clouds, too,
0:27:40 > 0:27:44could be alive with microorganisms.
0:27:44 > 0:27:46I'll just have a word with the pilot,
0:27:46 > 0:27:48without falling out of the airship!
0:27:51 > 0:27:54Dave, the deeper we are into thick cloud the better.
0:27:54 > 0:27:56I know there are limits to what you can do
0:27:56 > 0:27:59but that's what I'm looking for.
0:27:59 > 0:28:03The tricky thing is distinguishing between bacteria
0:28:03 > 0:28:06and other tiny particles like soot and dust
0:28:06 > 0:28:09which have been swept into the atmosphere by the wind.
0:28:12 > 0:28:16So Jim has rigged the airship with a particle analyser
0:28:16 > 0:28:18known as a WIBS machine.
0:28:21 > 0:28:27The WIBS machine uses lasers to detect soot, particles,
0:28:27 > 0:28:29to look for signs of life.
0:28:29 > 0:28:32You know when you go into a room and there's ultraviolet light,
0:28:32 > 0:28:36like a nightclub? Bits of dandruff, and your teeth,
0:28:36 > 0:28:40and even if you have a cup of urine - bit unlikely,
0:28:40 > 0:28:43but that would glow under the ultraviolet light.
0:28:43 > 0:28:45So there are these fluorescent molecules,
0:28:45 > 0:28:49that when you shine particular light on them, they glow.
0:28:49 > 0:28:51And that's essentially what this machine is going to look for.
0:28:51 > 0:28:55It's going to shine a laser at all the stuff that comes into it
0:28:55 > 0:28:58and if something glows, it's probably biological.
0:29:13 > 0:29:19As they enter the cloud, an inlet pipe draws in air for analysis.
0:29:26 > 0:29:29This is great. This is the thickest cloud we've been in, I think.
0:29:29 > 0:29:31This is the thickest cloud I'VE been in.
0:29:36 > 0:29:39So the first question is, are we detecting any signs
0:29:39 > 0:29:44that there might be microscopic life up in cloud vapour?
0:29:51 > 0:29:54Yeah, I'm actually seeing some response now.
0:29:54 > 0:29:58This top channel is one of the fluorescence channels
0:29:58 > 0:30:01and it responds to proteins.
0:30:01 > 0:30:05And anything above this baseline is actually fluorescence,
0:30:05 > 0:30:07and that's exactly what you are wanting to see.
0:30:07 > 0:30:13So you can see that we are getting fluorescence from material going in.
0:30:13 > 0:30:16These particles here, what size are they?
0:30:16 > 0:30:18The size is up to five microns,
0:30:18 > 0:30:22- nothing particularly big, quite small.- Smaller than pollen?
0:30:22 > 0:30:25Oh, much smaller than pollen, yeah, yeah.
0:30:25 > 0:30:28- And potentially the right size for bacteria?- Yes, yes.
0:30:28 > 0:30:31So this is quite...
0:30:31 > 0:30:33This is quite a big deal.
0:30:33 > 0:30:35To me, this is a really big deal.
0:30:35 > 0:30:39We've got evidence here that we've got bacteria in clouds
0:30:39 > 0:30:42and that's right at the cutting edge of science.
0:30:47 > 0:30:51Having established that some clouds are alive with bacteria,
0:30:51 > 0:30:54Chris now wants to know whether those microorganisms
0:30:54 > 0:30:57could be helping clouds to produce rain.
0:31:03 > 0:31:06Surprisingly, most rain starts as ice crystals,
0:31:06 > 0:31:08because high up inside clouds
0:31:08 > 0:31:12temperatures are often well below freezing.
0:31:12 > 0:31:17Those crystals of ice act like a magnet, attracting water vapour
0:31:17 > 0:31:18and growing rapidly.
0:31:20 > 0:31:24When they are big enough and heavy enough, they fall,
0:31:24 > 0:31:27and as they fall they melt to become rain.
0:31:30 > 0:31:34There is a theory that water freezes more easily
0:31:34 > 0:31:36around some types of particles than others.
0:31:41 > 0:31:47If you do the water and the mineral dust, I'll do the bacteria.
0:31:47 > 0:31:49So Chris is mounting an experiment
0:31:49 > 0:31:53to find out which is best at producing ice.
0:31:53 > 0:31:57Is it dust or bacteria?
0:31:59 > 0:32:04We've got three rows of drops here. We've got the first row near me
0:32:04 > 0:32:05is pure water,
0:32:05 > 0:32:08and then the second row
0:32:08 > 0:32:10has mineral dust in it,
0:32:10 > 0:32:14and the third row has bacteria that we know does live in clouds.
0:32:14 > 0:32:17And we are just going to drop the temperature on this plate
0:32:17 > 0:32:20and see which freezes more easily.
0:32:20 > 0:32:25And if a bacterial protein helps water turn into ice more easily
0:32:25 > 0:32:30than the mineral that we know is the most common reason that rain happens,
0:32:30 > 0:32:31that's really significant.
0:32:31 > 0:32:35You know, if that process is happening then bacteria
0:32:35 > 0:32:37might be making their own rain.
0:32:37 > 0:32:41This is the temperature of the plate cooling down.
0:32:41 > 0:32:44So it's just above freezing.
0:32:44 > 0:32:49So this is the pure water, this is the mineral dust,
0:32:49 > 0:32:51this is the bacteria.
0:32:53 > 0:32:56So we are below freezing.
0:32:56 > 0:33:00It's funny, isn't it? We talk about freezing as zero,
0:33:00 > 0:33:03but it's actually really hard to get water to freeze.
0:33:05 > 0:33:10In fact, pure water doesn't freeze until well below zero.
0:33:10 > 0:33:14There needs to be impurities in the water for it to freeze
0:33:14 > 0:33:16at higher temperatures.
0:33:16 > 0:33:19The water in your tap at home you make ice cubes from
0:33:19 > 0:33:22is full of all kinds of minerals
0:33:22 > 0:33:25and particles and dust and some bacteria.
0:33:25 > 0:33:29That means when you put it in the freezer it'll freeze.
0:33:29 > 0:33:32So we are at minus 3.9. So this is a cold day in there.
0:33:32 > 0:33:35That's minus 4.5 now.
0:33:41 > 0:33:44It's minus 8, almost minus 8.5.
0:33:46 > 0:33:47Nothing's frozen yet.
0:33:49 > 0:33:52- There you go.- There, there. - There you go.
0:33:52 > 0:33:56The whole lot just went. You just saw bang, bang, bang, bang, bang.
0:33:56 > 0:33:57Everything just froze.
0:34:02 > 0:34:04It's not a gradual thing. Once there's one ice crystal -
0:34:04 > 0:34:07bang, they all go.
0:34:07 > 0:34:11But that was only the bacterial ones. None of the mineral ones froze.
0:34:11 > 0:34:13Only when it is two degrees colder
0:34:13 > 0:34:16does the mineral dust finally start to freeze.
0:34:18 > 0:34:22- Those...- Yeah. Almost minus 11.
0:34:22 > 0:34:25Some of the mineral ones are going.
0:34:25 > 0:34:27Not only has the experiment demonstrated
0:34:27 > 0:34:30that ice forms around bacteria,
0:34:30 > 0:34:34but that it does so at a higher temperature than around dust.
0:34:42 > 0:34:47So the bacterial protein is more efficient than the main mineral
0:34:47 > 0:34:50that we think causes rain.
0:34:50 > 0:34:55And to me the key thing is here, bacteria have evolved a protein,
0:34:55 > 0:34:59they've made something that helps water freeze, that helps ice form.
0:35:01 > 0:35:04For Chris, this result challenges the whole way
0:35:04 > 0:35:07we understand how weather is created.
0:35:09 > 0:35:12It raises the intriguing possibility
0:35:12 > 0:35:15that living clouds will rain more readily than clouds that aren't.
0:35:17 > 0:35:21So knowing whether a cloud is a home to bacteria or not
0:35:21 > 0:35:24could help forecasters predict if it's going to rain.
0:35:26 > 0:35:29But the real enigma of living weather
0:35:29 > 0:35:32is why bacteria are in a cloud at all.
0:35:32 > 0:35:34Bacteria like moist environments.
0:35:34 > 0:35:38If you have rain, you have vegetation, that's food for bacteria.
0:35:38 > 0:35:40You know. Could it be that simple?
0:35:40 > 0:35:43That it's not just their way of getting out of the clouds,
0:35:43 > 0:35:45it's their way of creating an ecosystem
0:35:45 > 0:35:48in which in which they can live?
0:35:48 > 0:35:51That changes the whole way you've got to think about
0:35:51 > 0:35:54how weather happens on the planet.
0:36:03 > 0:36:07The team have come to the Florida Panhandle in search of rain,
0:36:07 > 0:36:09but now it's about to find them.
0:36:11 > 0:36:12Rain's coming.
0:36:12 > 0:36:14I can see it moving towards us.
0:36:16 > 0:36:18A powerful northerly wind has brought a cold front
0:36:18 > 0:36:211,000 miles long to the edge of the Gulf.
0:36:25 > 0:36:28The airship is lighter than air,
0:36:28 > 0:36:30its envelope filled with helium,
0:36:30 > 0:36:34and in these conditions it's hard to keep it under control.
0:36:43 > 0:36:47For a brief moment, Cloud Lab is at the mercy of the wind.
0:36:47 > 0:36:51Stranded on board is Dr Chris van Tulleken.
0:37:08 > 0:37:09That was terrifying.
0:37:09 > 0:37:11HE LAUGHS
0:37:11 > 0:37:14Here I am just sitting in the driving seat
0:37:14 > 0:37:17and the whole thing just turns on its end.
0:37:17 > 0:37:19Does that happen a lot?
0:37:19 > 0:37:22- That's the first time I've seen it go like that.- Oh, really?
0:37:22 > 0:37:26I assumed you knew what you were doing!
0:37:35 > 0:37:38With the airship finally secure, there's nothing to do
0:37:38 > 0:37:40but sit the storm out.
0:37:44 > 0:37:47You can hear it beating down on the top of the airship
0:37:47 > 0:37:50and it makes the hairs on the back of your neck go up.
0:37:50 > 0:37:53You can feel the energy in the air around you.
0:37:53 > 0:37:55It's absolutely fantastic.
0:37:55 > 0:37:57It's brilliant.
0:38:02 > 0:38:07In just a few hours, close to 30,000 tonnes of water
0:38:07 > 0:38:10are unleashed on this one small airfield alone.
0:38:12 > 0:38:16In the surrounding area, a staggering 2.8 million tonnes.
0:38:22 > 0:38:25With the passing of the cold front comes the opportunity
0:38:25 > 0:38:27for the team to pursue a new theme...
0:38:37 > 0:38:41..the relationship between the atmosphere and the life forms
0:38:41 > 0:38:42that make it their home.
0:38:45 > 0:38:49For all creatures that fly, the atmosphere is vital.
0:38:49 > 0:38:54It's a place to find food, to hunt and be hunted.
0:38:56 > 0:38:58But it's also a domain of wildly diverging
0:38:58 > 0:39:00and rapidly changing habitats,
0:39:00 > 0:39:06from gentle breezes to powerful thermals,
0:39:06 > 0:39:09which to us, remain mostly unseen.
0:39:11 > 0:39:14So the team want to examine the extent to which life actively
0:39:14 > 0:39:18exploits the many different characteristics of the atmosphere.
0:39:26 > 0:39:29This is Gulf Shores, Alabama.
0:39:29 > 0:39:31It's an important staging post
0:39:31 > 0:39:34for a number of different migratory bird species,
0:39:34 > 0:39:38all of which are trying to escape the approaching American winter.
0:39:41 > 0:39:45They are resting up here before the most perilous part of their journey
0:39:45 > 0:39:47to South and Central America,
0:39:47 > 0:39:50the 600 mile flight across the Gulf of Mexico.
0:39:58 > 0:40:00Andy is joining a group of scientists
0:40:00 > 0:40:04tracking the migration patterns of the birds that depart from here.
0:40:08 > 0:40:13They're on a dawn raid to catch and then tag some.
0:40:17 > 0:40:20- And you've got to check them every...?- 30 minutes.- OK.
0:40:22 > 0:40:25The question they're trying to answer is
0:40:25 > 0:40:27do the birds time their departures to take advantage
0:40:27 > 0:40:29of favourable atmospheric conditions?
0:40:35 > 0:40:40With the passing of the front, now is an ideal time to test the idea.
0:40:40 > 0:40:43- Three? So what's this? This is a Swainson's thrush?- No.
0:40:43 > 0:40:46No, it's a, er...
0:40:46 > 0:40:48- It's actually a warbler.- OK.
0:40:48 > 0:40:50- It that a migrant bird?- It is.
0:40:50 > 0:40:54They will come down all the way from Alaska and migrate through here.
0:40:54 > 0:40:57I really wanted to see a hummingbird. They just seem so delicate.
0:40:57 > 0:41:00Yeah, they're very, very delicate. That's why we put them in the bags
0:41:00 > 0:41:02instead of the boxes,
0:41:02 > 0:41:04just for that reason.
0:41:04 > 0:41:06The birds are caught between two conflicting pressures.
0:41:06 > 0:41:09On the one hand, winter is coming
0:41:09 > 0:41:11and they have to move before food becomes scarce.
0:41:14 > 0:41:17On the other, if they get their timing wrong,
0:41:17 > 0:41:20they may find themselves fighting headwinds.
0:41:27 > 0:41:30So, all the ones we are catching here, are they night-time migrants?
0:41:30 > 0:41:32Every one of all these birds are night migrants...
0:41:32 > 0:41:36Cloud Lab's biologist, Dr Sarah Beynon,
0:41:36 > 0:41:38has just joined the expedition.
0:41:41 > 0:41:45She's meeting the project's lead scientist, Professor Frank Moore.
0:41:47 > 0:41:50His team have caught more birds than usual.
0:41:50 > 0:41:54Do you think this is due to the weather that we had yesterday?
0:41:54 > 0:41:58I suspect that what happened was the birds that flew last night,
0:41:58 > 0:42:01they encountered that weather on the coast and they stopped here.
0:42:01 > 0:42:03And this is the last place they could stop
0:42:03 > 0:42:05before the Gulf of Mexico.
0:42:08 > 0:42:10Amongst the many species they are studying,
0:42:10 > 0:42:15some have migrating patterns that are still not fully understood,
0:42:15 > 0:42:17such as hummingbirds.
0:42:20 > 0:42:23He's making a spot there to attach the transmitter.
0:42:23 > 0:42:25The birds are fitted with radio transmitters
0:42:25 > 0:42:27to track their departure,
0:42:27 > 0:42:31and see if it is related to any particular weather conditions.
0:42:31 > 0:42:32How much does that weigh?
0:42:32 > 0:42:36It weighs about 4% of the bird's body mass.
0:42:36 > 0:42:38It may look invasive,
0:42:38 > 0:42:41but the procedures have been honed over many years.
0:42:41 > 0:42:44Where will you be picking up the data from that transmitter?
0:42:44 > 0:42:48From the towers that we have here on the peninsula,
0:42:48 > 0:42:50that will pick up the signal when the bird departs
0:42:50 > 0:42:52across the Gulf of Mexico.
0:42:53 > 0:42:56- Do you want to let him go? - Oh, yes, please.
0:42:56 > 0:42:58- OK, how do I hold him? - Open your hand.
0:43:01 > 0:43:04OK, and then hold the wings?
0:43:04 > 0:43:06If you just let your hands go
0:43:06 > 0:43:09- he'll fly off, or maybe with a little encouragement.- OK.
0:43:10 > 0:43:13- Good luck, little one.- There he goes.
0:43:13 > 0:43:15Wow! Pretty impressive.
0:43:15 > 0:43:17That was brilliant.
0:43:19 > 0:43:23Andy is discovering that some birds find it harder to leave than others.
0:43:27 > 0:43:29OK, this is not going well.
0:43:30 > 0:43:33Oh, man, you don't want to be the one bloke who kills
0:43:33 > 0:43:34a hummingbird on TV, do you?
0:43:38 > 0:43:41Phew! That was a close one.
0:43:41 > 0:43:42I'll have a little lie down now.
0:43:45 > 0:43:49Now all they can do is wait and see if the birds use the better weather
0:43:49 > 0:43:50to make the crossing that evening.
0:43:59 > 0:44:01On the airship, they're heading west along the Gulf
0:44:01 > 0:44:05so that Felicity can rendezvous with Sarah.
0:44:05 > 0:44:08Along the way, she'll gather more meteorological data
0:44:08 > 0:44:11to cross-reference with the bird tagging data.
0:44:19 > 0:44:22We put radio tags on some birds
0:44:22 > 0:44:25to see whether they actually made it across the Gulf,
0:44:25 > 0:44:30and three of the birds that we tagged made the journey.
0:44:30 > 0:44:34- And it took them between 16 and 24 hours.- Wow!
0:44:34 > 0:44:37But it just showed that they were able to make that journey.
0:44:40 > 0:44:44The tagged hummingbirds and thrushes departed that same evening
0:44:44 > 0:44:46and reached their destination.
0:44:52 > 0:44:55The passage of the cold front led to an improvement in the weather,
0:44:55 > 0:44:58and delivered a tailwind that the birds seem to have exploited.
0:45:01 > 0:45:05And the data Felicity has gathered suggests they're not the only birds
0:45:05 > 0:45:07that take advantage of a change in the wind.
0:45:10 > 0:45:13This is National Radar Data.
0:45:13 > 0:45:17So, any of the green, red and yellow signals you can see,
0:45:17 > 0:45:20that's bad weather that was sitting right on top of you
0:45:20 > 0:45:24and pinning all those birds down. But then as that front moves across,
0:45:24 > 0:45:29there's a sudden explosion of these sort of rosette blue colours.
0:45:29 > 0:45:31And nobody knew what they were at first,
0:45:31 > 0:45:35but now they know that it's biological matter showing up
0:45:35 > 0:45:40on the radar. So that is the birds leaving, it shows up on the radar.
0:45:42 > 0:45:46And if I just let this play, you can see that over the whole country
0:45:46 > 0:45:49as fronts move across,
0:45:49 > 0:45:53behind the fronts you'll see this sudden explosion of birds leaving.
0:45:58 > 0:46:03After the passage of a front, many millions of birds take to the skies
0:46:03 > 0:46:06in an attempt to reduce the energy required to make their migration.
0:46:09 > 0:46:13It really just shows how important these weather fronts are
0:46:13 > 0:46:16for the birds. They have to fly in the air
0:46:16 > 0:46:19that's following these cold fronts along.
0:46:21 > 0:46:25And just seeing it on this level shows that these weather fronts,
0:46:25 > 0:46:28you know, they are vital for movement,
0:46:28 > 0:46:31not just on a small scale but on a global scale.
0:46:47 > 0:46:54The team are heading further west to begin exploring their third theme -
0:46:54 > 0:46:58the relationship between the atmosphere and ourselves.
0:47:04 > 0:47:07Humans have been changing the atmosphere for millennia.
0:47:10 > 0:47:13In recent years we've witnessed the depletion of the ozone layer.
0:47:15 > 0:47:18Today our carbon emissions are changing the atmosphere
0:47:18 > 0:47:20on a global scale.
0:47:25 > 0:47:30The team want to explore one newly emerging and surprising consequence
0:47:30 > 0:47:34of our relationship with the atmosphere -
0:47:34 > 0:47:36the apparent increase in the frequency
0:47:36 > 0:47:39and intensity of hurricanes.
0:47:42 > 0:47:44They've arrived at New Orleans.
0:47:44 > 0:47:48In 2005, this was the scene of the deadliest hurricane
0:47:48 > 0:47:51to hit the United States in more than half a century.
0:47:54 > 0:47:57GEORGE W BUSH: Hurricane Katrina is now designated
0:47:57 > 0:47:59a Category 5 hurricane.
0:47:59 > 0:48:03We cannot stress enough the danger that this hurricane poses
0:48:03 > 0:48:07to Gulf Coast communities. I urge all citizens to put their own safety
0:48:07 > 0:48:12and the safety of their families first, by moving to safe ground.
0:48:14 > 0:48:17The city still bears the scars to this day.
0:48:20 > 0:48:23A lot of the damage is still so evident.
0:48:23 > 0:48:27There's foundations with nothing on them and roofs shaken to bits,
0:48:27 > 0:48:31and I can see a lot of houses where they're just destroyed.
0:48:35 > 0:48:39It really brings home how powerful this flooding must have been.
0:48:41 > 0:48:44Hurricanes have battered these shores since long before
0:48:44 > 0:48:46there were human settlements.
0:48:46 > 0:48:50It's a consequence of the particular geography in this area.
0:48:52 > 0:48:55As the team have learnt, the journey of water to sky
0:48:55 > 0:48:59releases vast amounts of energy through the action of latent heat.
0:49:02 > 0:49:06In the warm, shallow waters of the Gulf, that process takes place
0:49:06 > 0:49:10with such intensity it can help to generate a hurricane.
0:49:31 > 0:49:35But Katrina is evidence of a new and disturbing trend
0:49:35 > 0:49:39towards an increase in the number and intensity of storms.
0:49:41 > 0:49:45We already know that the sea surface temperatures
0:49:45 > 0:49:48drive the hurricanes, they're the hurricane fuel.
0:49:48 > 0:49:52And so if we look at a graph of sea surface temperatures,
0:49:52 > 0:49:57we can see that there's a very obvious upward trend.
0:49:57 > 0:50:01So temperatures are getting warmer and warmer, decade after decade.
0:50:01 > 0:50:06And that's what's driving not only more hurricanes but worse hurricanes.
0:50:06 > 0:50:08So what I'd like to know now
0:50:08 > 0:50:12is what's driving that upward trend in temperature?
0:50:18 > 0:50:21The most likely cause for the ocean warming is us.
0:50:24 > 0:50:29But Felicity suspects it may not be in the way we might at first expect.
0:50:34 > 0:50:37There's a newly emerging idea that the temperature of the Gulf
0:50:37 > 0:50:41may be influenced by pollutants in the atmosphere.
0:50:50 > 0:50:54To test the idea, Felicity is taking the airship on the eight-hour,
0:50:54 > 0:50:59300 mile journey to one of America's most industrialised cities -
0:50:59 > 0:51:01Houston, Texas.
0:51:13 > 0:51:16So, we've come to an area that has a lot of heavy industry
0:51:16 > 0:51:19and also one of the busiest shipping lanes in the US,
0:51:19 > 0:51:24because here we're likely to see what impact that's having
0:51:24 > 0:51:27on the clouds that are forming in this area.
0:51:29 > 0:51:32Clouds have an important effect on sea temperatures
0:51:32 > 0:51:35because of the way they block out the sun's heat.
0:51:35 > 0:51:38But the extent to which they block the sun
0:51:38 > 0:51:40depends upon what they're made from,
0:51:40 > 0:51:43because polluted clouds have different properties
0:51:43 > 0:51:45compared to clean clouds.
0:51:45 > 0:51:47What we'd now like to do
0:51:47 > 0:51:49is to try and get into some of these clouds over here.
0:51:49 > 0:51:52- We're looking for a dirty cloud. - Dirty?
0:51:52 > 0:51:55Something that's either over this shipping channel
0:51:55 > 0:51:58- or over the oil refineries.- OK.
0:52:09 > 0:52:13She first needs to confirm whether the cloud is polluted.
0:52:18 > 0:52:20OK, we're in.
0:52:27 > 0:52:31Jim detects methane and carbon dioxide -
0:52:31 > 0:52:33important markers for other pollutants.
0:52:39 > 0:52:43So, can we tell whether that was a dirty cloud or not?
0:52:43 > 0:52:46We can measure the cocktail of pollutants.
0:52:46 > 0:52:48So, what we've got here, we're getting these
0:52:48 > 0:52:50increases in concentration.
0:52:50 > 0:52:53So these lumps are where we went through clouds,
0:52:53 > 0:52:57and it's a peak in methane and carbon dioxide.
0:53:00 > 0:53:03The high levels of pollution mean that there are more particles
0:53:03 > 0:53:05on which the cloud droplets can form.
0:53:08 > 0:53:10And that has an important knock-on effect.
0:53:18 > 0:53:23This is the size distribution, and the average is about six microns
0:53:23 > 0:53:26and that's quite small. Whereas in the cleaner clouds,
0:53:26 > 0:53:28which we've flown through in Florida,
0:53:28 > 0:53:31- the average size is more like ten. - Right.
0:53:31 > 0:53:38- So we're seeing more small droplets that you would in a clean cloud?- Yes.
0:53:38 > 0:53:44In dirty clouds you have more and smaller particles,
0:53:44 > 0:53:47so they are going to be denser clouds, there's more droplets.
0:53:51 > 0:53:53The consequences of this are far-reaching.
0:53:56 > 0:53:59The more water droplets a cloud contains,
0:53:59 > 0:54:02the more sunlight it scatters and reflects.
0:54:05 > 0:54:09So less heat reaches the earth and the sea.
0:54:11 > 0:54:16The clouds here are dirty clouds, and because they're thicker and denser,
0:54:16 > 0:54:20they're blocking out more sunlight than clean clouds.
0:54:20 > 0:54:22So they're having a net cooling effect
0:54:22 > 0:54:24on the climate underneath them.
0:54:27 > 0:54:32So dirty clouds are cooling down temperatures.
0:54:33 > 0:54:34Right.
0:54:37 > 0:54:41It seems that polluted clouds cool the world's oceans.
0:54:44 > 0:54:48And yet sea surface temperatures are on the rise...
0:54:52 > 0:54:54..fuelling hurricanes.
0:54:59 > 0:55:03Felicity calls upon the one piece of data that can make sense
0:55:03 > 0:55:06of this confusing picture -
0:55:06 > 0:55:08the way in which pollution levels have changed
0:55:08 > 0:55:11over the past few decades.
0:55:11 > 0:55:12What I'm thinking
0:55:12 > 0:55:15is that the period when the atmosphere was at its dirtiest...
0:55:18 > 0:55:20And if you look at these hurricane seasons...
0:55:23 > 0:55:28..it's pretty much the same period of time when there were less hurricanes.
0:55:31 > 0:55:36- So it's possible that pollution is suppressing hurricanes.- Yeah.
0:55:40 > 0:55:44It's an extraordinary idea, that higher levels of pollution
0:55:44 > 0:55:48in the past might have been suppressing hurricanes,
0:55:48 > 0:55:52because polluted clouds were cooling the world's oceans.
0:55:54 > 0:55:57But environmental legislation
0:55:57 > 0:56:00has improved air quality across America.
0:56:04 > 0:56:07So there are fewer of these dense, polluted clouds.
0:56:11 > 0:56:14As a result, the seas have slowly warmed up again.
0:56:19 > 0:56:25So, what we're saying is that by cleaning up our atmosphere
0:56:25 > 0:56:28we have allowed there to be more hurricanes.
0:56:28 > 0:56:31So, we're not seeing an upward trend in hurricanes,
0:56:31 > 0:56:35what we HAVE seen in past decades when the air was dirty,
0:56:35 > 0:56:38was a suppression in hurricanes. So what we're seeing at the moment
0:56:38 > 0:56:41is a return to the natural state of things,
0:56:41 > 0:56:44a return to the normal number of hurricanes
0:56:44 > 0:56:47that you would expect to find in a season.
0:56:47 > 0:56:50And that's really...
0:56:50 > 0:56:53a fantastic story.
0:56:59 > 0:57:02Felicity has picked her way through the intricate evidence
0:57:02 > 0:57:07that might explain the rise in hurricane frequency and intensity.
0:57:13 > 0:57:16And the answer is as complex as it is surprising.
0:57:26 > 0:57:29The team have now completed the first half
0:57:29 > 0:57:31of their epic voyage across America.
0:57:48 > 0:57:53Next time, the team journey across the harsh desert of the west,
0:57:53 > 0:57:56and on to the Pacific Ocean.
0:57:56 > 0:57:58We've made it!
0:57:58 > 0:58:01Andy will take to the skies once again,
0:58:01 > 0:58:05searching for life at the edge of existence.
0:58:06 > 0:58:11Felicity and Jim will investigate our role in making rain.
0:58:14 > 0:58:17And Sarah experiences life on the wing.